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Date: Wed, 5 Dec 2012 16:46:39 +0100
From: Bruno Dupuis <python.ml.bruno.dupuis@lisael.org>
To: python-list@python.org
Subject: why does dead code costs time?
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Xref: csiph.com comp.lang.python:34288

Hi,

I'm interested in compilers optimizations, so I study python compilation process

I ran that script:

    import timeit

    def f(x):
        return None

    def g(x):
        return None
        print(x)

    number = 10000

    print(timeit.timeit('f(1)',setup="from __main__ import f", number=number))
    print(timeit.timeit('g(1)',setup="from __main__ import g", number=number))   

    print(dis.dis(f))
    print(dis.dis(g))

It gives this output:

    0.003460251959040761
    0.004164454061537981
     17           0 LOAD_CONST               0 (None) 
                  3 RETURN_VALUE         
    None
     20           0 LOAD_GLOBAL              0 (None) 
                  3 RETURN_VALUE         

     21           4 LOAD_GLOBAL              1 (print) 
                  7 LOAD_FAST                0 (x) 
                 10 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
                 13 POP_TOP              
    None

I do not understand why the dead code `print(x)` takes time (~20% in
that case). As we see in the opcode, a call to g(1) returns immediately, so
there should be no delay at all. Where am i wrong?

mmhh... it comes to me now that the gap must be in function loading time...
I'll check ceval.c

However, isn't there a room for a slight optim here? (in this case, the
dead code is obvious, but it may be hidden by complex loops and
conditions)

Cheers


-- 
Bruno Dupuis